1. Field of the Invention
This invention relates to fired heater operation, particularly fuel fired furnaces.
2. Description of the Prior Art
Fuel fired furnaces are in general any process heater (burner), boiler, steam super heater, and the like. This invention is applicable to all such apparatus (devices), but, for the sake of clarity and brevity, will be described here in terms of a commercial, hydrocarbon processing plant, and the type of furnace normally employed in such a plant, e.g., a refinery or olefin plant.
A furnace normally is powered by a combustible fuel such as natural gas. As such, the furnace requires an ample supply of combustion oxygen which is usually supplied in the form of ambient air that is forced into the heater section of the furnace. The heater section contains, in simplest form, one or more burners, usually four, which pick up combustion air from an air plenum. Ambient air is supplied to the plenum by a very large blower that forces air through an air duct into the plenum. The burners pick up the combustion air needed, mix the air with the fuel and combust the mixture at the burner tip.
If the burners stop combustion or the blower stops operating, the entire furnace is shut down, and, possibly, the whole plant is shut down. Furnace shutdown has a number of consequences and ramifications.
There are a number of operating aspects for a furnace. First of all is start up (warm up). It can take a very long time to get a furnace and its vent stack up to normal operating temperatures. It is desirable to achieve warm up as quickly as possible, and to avoid shutdowns that would require start up all  over again. If a furnace is shut down, it is desirable to be able to achieve start up again as quickly as possible and in as short a time as possible. This means not letting the furnace cool down any more than absolutely necessary.
Next is normal operation. After start up, it is desirable to keep the furnace operating at all times to maintain plant production at least at some, if not peak, producing capacity. Avoiding complete furnace shutdown and cool off is highly desirable, even when a major piece of the furnace such as the air blower becomes inoperable for any reason such as loss of power or mechanical failure.
Safety is always a consideration in furnace operation. There are well-known safety concerns involved in shutting down and restarting a furnace that are not present in the normal operation of the furnace. Accordingly, from a safety aspect it is better to keep the furnace running, even at reduced capacity, than to shut it down and then restart it after it has cooled down.
There is also the possibility of an emergency such as the air blower (either the forced draft or the induced draft blower) losing its power source which then causes the burners to shut down. If the furnace can be kept operating, even at reduced capacity, during an emergency this avoids complete cool off and restart of the furnace thus avoiding some safety considerations and a long warm up period on restart.
Finally, during shutdown for maintenance work on the blower, it is desirable to keep the furnace in operation, thereby maintaining plant production and avoiding furnace cooling and restarting.
This invention addresses all these considerations in that it provides for the continued operation of a furnace at a reduced, but acceptable level of production capacity, thereby avoiding the complete cooling of the furnace and the need for a full start up cycle. With this invention normal operation at some significant level, e.g., at least 66% of normal production capacity can be maintained, but if shutdown does become necessary, this invention can help achieve a quicker start up. By avoiding at least some shutdowns, this invention  helps minimize safety considerations brought on by restarting a cold furnace. This invention also helps minimize complete furnace shutdown and start up delays in certain emergencies such as blower failure.